Manages scheduler-initiated background jobs. A background scheduler is always started
by one of the servers in a cluster. When a time arrives for a job to be invoked,
the scheduler publishes a message on a broker, the message is picked up
by one of the servers and a service is invoked.

Extra data is a string which Zato doesn’t interfere with - it’s provided as is
to a service in its request’s raw_request
attribute.